[devmgr] Fix race in serial console start

If the system was booting slowly (e.g. because ASAN + emulation), the
serial console might not be started.

Test: Booted in qemu-arm64 with ASAN and some prints enabled to record
how long it took for /dev/misc/console to come up.  Observed that
console connected even if it took over 3s for the console device to come
up.

Change-Id: Ibec19f4db78c7641cd31e0b817024d2cb3185252